1. Field of the Invention
The present invention relates to a purification process for cyclic formals which are useful as solvents, intermediates of drugs, starting materials for resins, and the like. More particularly, it relates to an economically advantageous purification process for obtaining cyclic formals of high purity which contains only a very small amount of water, in which water is efficiently removed from a mixture of a cyclic formal and water which is difficult to be separated from the mixture because of the azeotropy between cyclic formal and water.
2. Description of Related Art
Cyclic formals typified by 1,3-dioxolan, 1,4-butanediol formal, diethylene glycol formal, methyl-1,3-dioxolan, etc. are known to be obtainable from cyclizing reactions between a corresponding glycol and an aldehyde, and between a corresponding alkylene oxide and an aldehyde. For example, concerning a method for preparing a typical cyclic formal, 1,3-dioxolan, German patent No. 1914209 discloses a process for preparing it by reacting glycol with formaldehyde in the presence of an acid catalyst, and Ind. Eng. Chem., 46,787 (1954) and U.S. Pat. No. 3,857,759 both disclose a process for preparing 1,3-dioxolan by reacting glycol and paraformaldehyde in the presence of an acid catalyst.
These processes for preparing cyclic formals which employ a glycol and an aldehyde as starting materials involve drawbacks in that the cyclic formal produced and a by-produced water or water which is present in a form of an aqueous aldehyde solution often co-boil (azeotropy), thereby rendering separation of water difficult by ordinary distillation steps.
Taking 1,3-dioxolan as an example, the above mentioned German patent No. 1914209 describes that as much as 7% of water is contained. In order to obtain 1,3-dioxolan of high purity by removing water from a mixture of 1,3-dioxolan and water, the above-mentioned Ind. Eng. Chem., 46,787 (1954) discloses a process in which a reaction distillate containing 1,3-dioxolan and water is added with sodium chloride for phase separation into two phases, and the organic phase is subjected to a purifying distillation for purification, while U.S. Pat. No. 3,857,759 discloses a process in which a reaction distillate is added with cyclohexane before purification. However, the former is not suitable as an industrial purification process, and the latter raises a problem in that water cannot be separated sufficiently for obtaining 1,3-dioxolan of high purity.
These phenomena do not specifically occur only in processes for preparing 1,3-dioxolan, but are common in processes for obtaining cyclic formals which form an azeotropic system with water. Accordingly, an economical purification process for obtaining cyclic formals of high purity by which water is efficiently removed from a mixture of a cyclic formal and water has still been desired.
Under the above circumstances, the present inventors have carried out extensive studies in order to solve the aforementioned problems. Based on the phenomenon that water co-boils together with various solvents, which is called azeotropy between water and solvents, they have carefully studied on physicochemical properties of cyclic formals, water, azeotropes between cyclic formals and water, etc., and, as a result, have found that if n-pentane is added to a distillation system of a crude cyclic formal containing water, it destroys the azeotropic system between the cyclic formal and water to effectively distill off water together with n-pentane, thereby yielding 1,3-dioxolan of high purity as a bottom liquid of the distillation tower, and that this method is excellent in view of operation, leading to completion of the present invention.